Try using cookies. E.g.,
Response.Cookies["username"].Value=textBoxUsername.Text;
Response.Cookies["username"].Expires=DateTime.Now.AddDays(30);
This is one way. You can also use
HttpCookie cookie=new HttpCookie();
cookie.Value=textBoxUsername.Text;
cookie.Expires=DateTime.Now.AddDays(30);
Response.Cookies.Add(cookie);
Those are simple ways of using cookies. In my examples, the cookie expires 30 days from creation.